JuliaGPU · anicusan · Jul 22, 2025 · Jul 1, 2025 · Jun 15, 2025 · Jul 1, 2025
diff --git a/Project.toml b/Project.toml
@@ -8,20 +8,19 @@ ArgCheck = "dce04be8-c92d-5529-be00-80e4d2c0e197"
 GPUArraysCore = "46192b85-c4d5-4398-a991-12ede77f4527"
 KernelAbstractions = "63c18a36-062a-441e-b654-da1e3ab1ce7c"
 Markdown = "d6f4376e-aef5-505a-96c1-9c027394607a"
+UnsafeAtomics = "013be700-e6cd-48c3-b4a1-df204f14c38f"
 
 [weakdeps]
-Metal = "dde4c033-4e86-420c-a63e-0dd931031962"
 oneAPI = "8f75cd03-7ff8-4ecb-9b8f-daf728133b1b"
 
 [extensions]
-AcceleratedKernelsMetalExt = "Metal"
 AcceleratedKernelsoneAPIExt = "oneAPI"
 
 [compat]
 ArgCheck = "2"
 GPUArraysCore = "0.2.0"
 KernelAbstractions = "0.9.34"
 Markdown = "1"
-Metal = "1"
-oneAPI = "1, 2"
+UnsafeAtomics = "0.3.0"
 julia = "1.10"
+oneAPI = "1, 2"
diff --git a/ext/AcceleratedKernelsMetalExt.jl b/ext/AcceleratedKernelsMetalExt.jl
diff --git a/src/AcceleratedKernels.jl b/src/AcceleratedKernels.jl
@@ -14,6 +14,7 @@ module AcceleratedKernels
 using ArgCheck: @argcheck
 using GPUArraysCore: AnyGPUArray, @allowscalar
 using KernelAbstractions
+import UnsafeAtomics
 
 
 # Exposed functions from upstream packages

diff --git a/src/accumulate/accumulate.jl b/src/accumulate/accumulate.jl
@@ -40,7 +40,7 @@ include("accumulate_nd.jl")
         min_elems::Int=2,
 
         # Algorithm choice
-        alg::AccumulateAlgorithm=DecoupledLookback(),
+        alg::AccumulateAlgorithm=ScanPrefixes(),
 
         # GPU settings
         block_size::Int=256,
@@ -60,7 +60,7 @@ include("accumulate_nd.jl")
         min_elems::Int=2,
 
         # Algorithm choice
-        alg::AccumulateAlgorithm=DecoupledLookback(),
+        alg::AccumulateAlgorithm=ScanPrefixes(),
 
         # GPU settings
         block_size::Int=256,
@@ -89,13 +89,13 @@ becomes faster if it is a more compute-heavy operation to hide memory latency -
 
 ## GPU
 For the 1D case (`dims=nothing`), the `alg` can be one of the following:
-- `DecoupledLookback()`: the default algorithm, using opportunistic lookback to reuse earlier
-  blocks' results; requires device-level memory consistency guarantees, which Apple Metal does not
-  provide.
-- `ScanPrefixes()`: a simpler algorithm that scans the prefixes of each block, with no lookback; it
-  has similar performance as `DecoupledLookback()` for large block sizes, and small to medium arrays,
+- `ScanPrefixes()`: the default algorithm that scans the prefixes of each block, with no lookback; it
+  has better performance than `DecoupledLookback()` for large block sizes, and small to medium arrays,
   but poorer scaling for many blocks; there is no performance degradation below `block_size^2`
-  elements.
+  elements, but it remains fast well into millions of elements.
+- `DecoupledLookback()`: a more complex algorithm using opportunistic lookback to reuse earlier
+  blocks' results; requires device-level memory consistency guarantees (which Apple Metal does not
+  provide) and atomic orderings; theoretically more scalable for many blocks.
 
 A different, unique algorithm is used for the multi-dimensional case (`dims` is an integer).
 
@@ -105,13 +105,7 @@ The temporaries are only used for the 1D case (`dims=nothing`): `temp` stores pe
 `temp_flags` is only used for the `DecoupledLookback()` algorithm for flagging if blocks are ready;
 they should both have at least `(length(v) + 2 * block_size - 1) ÷ (2 * block_size)` elements; also,
 `eltype(v) === eltype(temp)` is required; the elements in `temp_flags` can be any integers, but
-`Int8` is used by default to reduce memory usage.
-
-# Platform-Specific Notes
-On Metal, the `alg=ScanPrefixes()` algorithm is used by default, as Apple Metal GPUs do not have
-strong enough memory consistency guarantees for the `DecoupledLookback()` algorithm - which
-produces incorrect results about 0.38% of the time (the beauty of parallel algorithms, ey). Also,
-`block_size=1024` is used here by default to reduce the number of coupled lookbacks.
+`UInt8` is used by default to reduce memory usage.
 
 # Examples
 Example computing an inclusive prefix sum (the typical GPU "scan"):
@@ -123,7 +117,7 @@ v = oneAPI.ones(Int32, 100_000)
 AK.accumulate!(+, v, init=0)
 
 # Use a different algorithm
-AK.accumulate!(+, v, alg=AK.ScanPrefixes())
+AK.accumulate!(+, v, alg=AK.DecoupledLookback())
 ```
 """
 function accumulate!(
@@ -160,8 +154,6 @@ function _accumulate_impl!(
     dims::Union{Nothing, Int}=nothing,
     inclusive::Bool=true,
 
-    # FIXME: Switch back to `DecoupledLookback()` as the default algorithm
-    #         once https://github.com/JuliaGPU/AcceleratedKernels.jl/pull/44 is merged.
     alg::AccumulateAlgorithm=ScanPrefixes(),
 
     # CPU settings
@@ -214,7 +206,7 @@ end
         min_elems::Int=2,
 
         # Algorithm choice
-        alg::AccumulateAlgorithm=DecoupledLookback(),
+        alg::AccumulateAlgorithm=ScanPrefixes(),
 
         # GPU settings
         block_size::Int=256,

diff --git a/src/accumulate/accumulate_1d_gpu.jl b/src/accumulate/accumulate_1d_gpu.jl
@@ -169,9 +169,9 @@ end
     running_prefix = prefixes[iblock - 0x1 + 0x1]
     inspected_block = signed(typeof(iblock))(iblock) - 0x2
     while inspected_block >= 0x0
-
         # Opportunistic: a previous block finished everything
-        if flags[inspected_block + 0x1] == ACC_FLAG_A
+        if UnsafeAtomics.load(pointer(flags, inspected_block + 0x1), UnsafeAtomics.monotonic) == ACC_FLAG_A
+            UnsafeAtomics.fence(UnsafeAtomics.acquire) # (fence before reading from v)
             # Previous blocks (except last) always have filled values in v, so index is inbounds
             running_prefix = op(running_prefix, v[(inspected_block + 0x1) * block_size * 0x2])
             break
@@ -194,11 +194,17 @@ end
     end
 
     # Set flag for "aggregate of all prefixes up to this block finished"
-    @synchronize()      # This is needed so that the flag is not set before copying into v, but
-                        # there should be better memory fences to guarantee ordering without
-                        # thread synchronization...
+    # There are two synchronization concerns here:
+    # 1. Withing a group we want to ensure that all writed to `v` have occured before setting the flag.
+    # 2. Between groups we need to use a fence and atomic load/store to ensure that memory operations are not re-ordered
+    @synchronize() # within-block
+    # Note: This fence is needed to ensure that the flag is not set before copying into v.
+    #       See https://doc.rust-lang.org/std/sync/atomic/fn.fence.html
+    #       for more details.
+    #       We use the happens-before relation between stores to `v` and the store to `flags`.
+    UnsafeAtomics.fence(UnsafeAtomics.release)
     if ithread == 0x0
-        flags[iblock + 0x1] = ACC_FLAG_A
+        UnsafeAtomics.store!(pointer(flags, iblock + 0x1), convert(eltype(flags), ACC_FLAG_A), UnsafeAtomics.monotonic)
     end
 end
 
@@ -285,7 +291,7 @@ function accumulate_1d_gpu!(
     end
 
     if isnothing(temp_flags)
-        flags = similar(v, Int8, num_blocks)
+        flags = similar(v, UInt8, num_blocks)
     else
         @argcheck eltype(temp_flags) <: Integer
         @argcheck length(temp_flags) >= num_blocks

diff --git a/src/arithmetics.jl b/src/arithmetics.jl
@@ -307,7 +307,7 @@ end
         dims::Union{Nothing, Int}=nothing,
 
         # Algorithm choice
-        alg::AccumulateAlgorithm=DecoupledLookback(),
+        alg::AccumulateAlgorithm=ScanPrefixes(),
 
         # GPU settings
         block_size::Int=256,
@@ -318,9 +318,6 @@ end
 Cumulative sum of elements of an array, with optional `init` and `dims`. Arguments are the same as
 for [`accumulate`](@ref).
 
-## Platform-Specific Notes
-On Apple Metal, the `alg=ScanPrefixes()` algorithm is used by default.
-
 # Examples
 Simple cumulative sum of elements in a vector:
 ```julia
@@ -360,7 +357,7 @@ end
         dims::Union{Nothing, Int}=nothing,
 
         # Algorithm choice
-        alg::AccumulateAlgorithm=DecoupledLookback(),
+        alg::AccumulateAlgorithm=ScanPrefixes(),
 
         # GPU settings
         block_size::Int=256,
@@ -371,9 +368,6 @@ end
 Cumulative product of elements of an array, with optional `init` and `dims`. Arguments are the same
 as for [`accumulate`](@ref).
 
-## Platform-Specific Notes
-On Apple Metal, the `alg=ScanPrefixes()` algorithm is used by default.
-
 # Examples
 Simple cumulative product of elements in a vector:
 ```julia

diff --git a/test/runtests.jl b/test/runtests.jl
@@ -21,7 +21,9 @@ elseif "--oneAPI" in ARGS
     using oneAPI
     oneAPI.versioninfo()
     const BACKEND = oneAPIBackend()
-    TEST_DL[] = true
+
+    # FIXME: need atomic orderings for `DecoupledLookback` in oneAPI
+    # TEST_DL[] = true
 elseif "--AMDGPU" in ARGS
     Pkg.add("AMDGPU")
     using AMDGPU